Apple is now selling and renting digital 4K movies via iTunes, in addition to its HD and SD resolution versions. The addition of 4K content, which you can stream as well as download, is mainly designed to support the new Apple TV, which supports 4K HDR output for the first time. Plus, if you bought HD versions of titles for which 4K HDR is available, Apple is automatically upgrading them for free.
This is a key piece of Apple's incentive puzzle for getting users to upgrade to a new Apple TV, as it means there will be a strong content pool that users can access right away. Apple will also be able to take advantage of 4K streaming content provided via Netflix, which has offered both 4K and HDR streaming on other platforms for a while now, and Amazon Prime Video, which is finally arriving on the platform as previously announced.
Apple's store isn't the first digital movie storefront to offer 4K versions of movies -- Google Play Movies & TV introduced 4K and HDR videos in July for streaming customers in the U.S. and Canada. This is supported on compatible Android TV television sets and streaming devices, including the Nvidia Shield. But Apple's pricing of 4K versions at the same rate as HD titles is a smart competitive move.
iTunes, however, likely has a far greater reach than Google Play, as it's been a destination for digital sales and rentals since long before Google even began offering those services to customers.